Background: Remodeling of pulmonary arteries is a key feature in the pathogenesis of pulmonary arterial hypertension (PAH); smooth muscle cell proliferation and survival is essential to this vascular remodeling. It has previously been described that pulmonary artery vascular smooth muscle cells (PAVSMCs) have a metabolic glycolytic shift similar to that seen in cancer cells. Additionally, it is known that cell proliferation is suppressed by inhibition of mammalian target of rapamycin complex 1 (mTORC1). mTOR also acts via mTORC2, which may serve as a key coordinator of glycolytic shift and cell proliferation in PAVSMCs. This study aimed to explore the role of mTOR signaling in PAVSMC proliferation and survival in idiopathic PAH (iPAH).

Methods and Results: Lung tissue was obtained from 4 non-diseased controls and 4 idiopathic PAH patients. PAVSMCs isolated from the idiopathic PAH patient demonstrated activated mTORC1 and mTORC2 pathways. Even after serum deprivation (i.e. in the absence of mitogenic stimuli) iPAH PAVSMCs had aberrant increases in proliferation and survival. iPAH PAVSMCs had a 2-fold higher ATP content which was substantially reduced by a glycolytic inhibitor. ATP content was not impacted as robustly by a mitochondrial respiratory chain inhibitor. The glycolytic inhibitor also decreased proliferation and promoted apoptosis in iPAH PAVSMCs. By using small interfering RNAs (siRNAs), the authors demonstrated that mTORC2 is required for increases in cellular ATP. To further detail molecular mechanisms underlying cellular proliferation and mTORC2, experiments were performed to investigate the role of energy sensor AMP-activated protein kinase (AMPK) as well as NADPH oxidase 4 (Nox4), which is known to be a regulator of pulmonary vascular remodeling in PAH. They found that AMPK signaling is downregulated in iPAH PAVSMCs and that mTORC2 inhibition of AMPK leads to mTORC1 activation, allowing proliferation of the vascular smooth muscle cells. Nox4 was markedly increased in iPAH PAVSMCs, and it was demonstrated that Nox4 acts as a regulator of mTORC2 signaling on proliferation and survival of PAVSMCs. In rats exposed to chronic hypoxia, there was upregulation of mTOR and Nox4 by day 2, which preceded vascular remodeling. In this rat model, an inhibitor of mTOR kinase (PP242) induced smooth muscle cell apoptosis, decreased medial wall thickness, and improved pulmonary vascular density on micro-CT.

Conclusions: These in vivo animal and in vitro human experiments identify mTORC2 as a key player in the pathogenesis of pulmonary vascular remodeling in idiopathic PAH. mTORC2 appears to be an important regulator of glycolysis-dependent PAVSMC proliferation and survival. The data suggest that AMPK and Nox4 play a role in the molecular mechanism of mTORC2’s action. As drug investigation in PAH is moving away from vasodilation and towards modification of pulmonary vascular remodeling, mTORC2 may be a potential therapeutic target, particularly because inhibition of mTOR kinase appeared to be selective for diseased PAVSMCs.

Expert Commentary: Despite the availability of a wide range of therapeutic agents with vasodilatory properties, none of these have been shown to be capable of either halting or reversing established vascular remodeling in patients with PAH. To increase our chances of discovering novel therapeutics capable of addressing this gap in our capacity to alter the natural history of PAH, it is mandatory to understand the key molecular mechanisms that drive abnormal cellular response responsible for the pulmonary vasculopathy.

The paper by Goncharov et al provides exciting evidence using both cell based and animal models for a critical role for mTORC2, a signaling mediator of the mTOR pathway, in promoting uncontrolled PASMC proliferation and survival. These comprehensive pre-clinical studies provide a strong rationale for pursuing this target in future drug development efforts as it may help uncover small molecules capable of treating vascular remodeling by suppressing proliferation and inducing apoptosis of PASMC via specific inhibition of mTORC2. A major challenge in translating these findings into effective PAH therapeutics will be the identification of compounds capable of specifically targeting mTORC2 in PASMCs since most of the available mTOR modulators have nonspecific profiles and could result in significant side effects. This is to be expected when considering the fundamental role that the mTOR pathway plays in cell metabolism and the ongoing work done in the cancer field to develop effective anti-neoplastic mTOR modulators. Nevertheless, the findings in this paper should be viewed as a step forward in our understanding of PAH pathobiology and should pave the ground for efforts to develop novel therapeutics that will hopefully be effective in changing the natural history of this devastating disease.

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